Question

20. A 55-year-old near-sightedness person wears spectacles with a power of 2.5 D for distance viewing. His doctor prescribes a correction of + 2.0 D in the near-vision section of his bi-focal. This is measured relative to the main part of the lens. Select the correct statements: The focal length of distance-viewing part of the lensis - 40 cm. (1) The focal length of the near vision part of the lens is + 50 cm. (ii) The focal length of distance-viewing part of the lens is + 50 cm. (iv) The focal length of near-vision part of the lens is -40 cm. [A] Both ) and (11) [B] Both ) and (iv) [C] Both (ii) and (iii) [D] Both (i) and (iv)​

Answer 1

Answer:

Explanation:

Option c is the correct option.

Answer 2

Answer:

a. Both i) and ii)

Explanation:

for near sightedness,

p= - 2.5 D

p= 1/f

f =1/ -2.5

f =-0.4 m

f =-40 cm(1m =100 cm)

for far sightedness ,

given ,

p= +2.0D

p= 1/f

f =1/2.0

f =+ 0.5 m

f = +50 cm

hence, the focal length of distance viewing part of the lens is 40 cm and the focal length of near vision part of lens is 50 cm.